Examples of powdery or fragmental materials are different fibers, chippings and wood chips used in the manufacture of chipboard or fiberboard and the like. In the manufacture of such boards, increasing use is being made of waste material. This has led to a need to remove impurities from the materials used for board manufacture. Such impurities include various minerals, rocks, sand, etc. Solutions are known in which impurities are separated from materials by merely using an air current. These solutions have the drawbacks of high energy consumption and dust emissions. Moreover, in purification based on the use of a gas flow, fine impurities cannot be removed as desired, leading to an unsatisfactory purification result.
In mineral separation technology, a known method is dry jigging or pulse separation. In pulse separation, short gas impacts are applied from below to material flowing on a carrier surface pervious to gas. The lifting effect of the gas impact on a heavier particle is smaller than on a lighter particle because of the lower acceleration of the former. Therefore, the lighter particles, which have risen higher during the gas impact, come down more slowly during the intermission and are concentrated in the top part of the material layer. The heavier particles are concentrated in the bottom part of the layer. To separate the layers, they must be moved from the input end of the carrier surface towards its output end. The movement is achieved e.g. by using directional vibration, and the separation is performed e.g. at the output end by using a separating knife or, before it, a screw that moves the bottom layer to one side of the apparatus. The separation of the aforesaid layers has been determined according to the highest mineral quantity. In this case, the mineral content of the bottom layer is usually only 10-50%, which means that further enrichment is required. Different materials present different requirements regarding the gas impact/intermission ratio, pulse number and impact intensity. In known devices, a blast apparatus, a rotary valve and piping and gas distribution below the plane are not applicable for the separation of finegrained minerals. The large volume of such gas apparatus interferes with the advance of fast pulses to the separation plane, so they are only applicable for rough separation.
The problem is how to achieve a sharp gas impact and a high pulse number uniformly e.g. on a large surface.